Should I be worried that she didn't message me back?

[James]

I did some little flirting with her on facebook for like a week (I liked her new profile picture, we had a poke war, commenting random google images on her wall for like an hour lol, etc) and then Monday she poked me, and I poked her back, but now she hasn't poked me again since the (she's usually almost never on during the school week though, last week was weird). I wanted to ask her to hang out this weekend sometime, so then two days ago (Thursday) I saw her on and I said "hey loser" to her on chat, but she didn't reply and it turns out she didn't even see it until last night (Friday). Should I worry that she didn't message me back? I mean I wouldn't message someone back a day late if they just said "hey," so I don't think so. And should I try messaging her again tonight or should I wait a day or two longer?
Thanks
PS I know that just because I flirted a bit with her doesn't make her mine for the taking, but it just seemed weird that she abruptly stopped all contact. And I know that this should be a pretty insignificant thing to worry about, but I just really like her, I guess.

Oh yeah and a few weeks ago something similar happened where I sent her something and she didn't reply and then that time she replied 5 days later. Although that actually had something to say, it wasn't just "hey" (I sent her a picture of a turtle and told her it looked like her...she seemed to think it was really funny. That was the first time I really flirted with her.)

[Abbey]

Ok, don't freak her out with all your flirtiness, because she probably is thinking that your one of those clingy types. If she really likes you, then she would have messaged you back or messaged you first. But theirs that possibility that she's busy or something. I think you should wait a day or 2 to message her back, and then when you message her be like "hey, how are you?" cause you don't wanna ask her out right away and her give you some lame excuse to not go. Have a few normal conversations with her first, and figure out her personality a little better.